#! /usr/bin/perl
use strict;
use feature qw{ say };
use warnings;

use XML::LibXML;

my $xml = '<header>
   <id x_id="1">
      <a></a>
      <b></b>
      <c>NA</c>
   </id>
</header>';

print "Please specify node c content: ";
chomp( my $new_text = <STDIN> );

my $dom = 'XML::LibXML'->load_xml(string => $xml);
for my $node($dom->findnodes('(/header/id/b | /header/id/c)')) {
    $node->removeChildNodes;
    $node->appendText($new_text);
}
print $dom;